A partial meltdown in the unit is "highly possible," he told reporters. "Because it's inside the reactor, we cannot directly check it but we are taking measures on the assumption of the possible partial meltdown."

He also said a hydrogen explosion could occur at the reactor, Unit 3 of the Fukushima Daiichi nuclear complex. That would follow a Saturday blast at Unit 1 of the same complex as operators attempted to prevent a nuclear meltdown by injecting sea water into it.

Japan's largest electric utility Sunday released what it said was slightly radioactive air at Unit 3.

Both reactors are among three shut down Friday at the Fukushima Daiishi plant when their cooling functions stopped after the area was jolted by a magnitude 8.9 earthquake.

Tokyo Electric Co., or TEPCO, Saturday began pouring seawater and boric acid into its Fukushima Daiichi power plant Unit 1 reactor, whose core partially melted. On Sunday TEPCO released air containing radioactive materials for more than 2 hours and injected water at the Unit 3 nuclear reactor container vessel to reduce pressure and temperature to save the reactor from a possible meltdown.

Critical core cooling systems failed at both reactors. The Unit 2 reactor, though shut down by the quake, was not in the same trouble.

Released steam raised radiation levels above safety limits outside the Unit 3 reactor Sunday, TEPCO officials said, adding they informed the government of an "emergency situation." Still, they said, there was no immediate threat to human health.

Edano said the radiation levels later fell.

Tepco
/
Reuters

The damaged roof of reactor No. 1 at the Fukushima Daiichi nuclear plant after an explosion that blew off the upper part of the structure is seen in this Saturday photo released by Tokyo Electric Power Company (TEPCO) in Fukushima Prefecture, northeastern Japan.

A similar radiation increase was seen Saturday during venting at the No. 1 reactor before it exploded. The blast destroyed the exterior walls of a building, but did not breach the steel housing enveloping the reactor there, officials said.

Radioactive cesium and iodine were detected near the facility Saturday, indicating that the melting had occurred, Kyodo News Service reported.

Officials said that at one point, the Unit 1 was releasing each hour the amount of radiation a person normally absorbs from the environment each year.

It is the first time a Japanese nuclear plant has ever experienced any level of reactor core melting.

TEPCO on Sunday also said it was preparing pressure reducing measures for its nearby Fukushima Daini Nuclear Power Station, where four reactors were shut down due to earthquakes. Releases of air with radioactive material were being considered for Daini units 1, 2, and 3.

Earlier Edano said TEPCO has begun new cooling operations to fill the Daiishi No. 1 reactor where the melting occurred with seawater and pour in boric acid, which absorbs neutrons, an operation expected to take several hours, Kyodo reported.

Filling the entire reactor container with seawater will take about 10 days, Edano said. It is likely that the reactor will have to be decommissioned because of the contamination by salts and other substances, experts said, according to Kyodo.

Edano on Sunday said the cooling operation at Unit 1 was going smoothly.

Robert Alvarez, senior scholar at the Institute for Policy Studies and former senior policy adviser to the U.S. secretary of energy, said in a briefing for reporters that the seawater was a desperate measure.

"It's a Hail Mary pass," he said.

He said that the success of using seawater and boron to cool the reactor will depend on the volume and rate of their distribution.

The evacuation zone around the facilities was doubled in size from 6 miles to 12 miles. More than 210,000 people had been evacuated from the area by early Sunday morning, and evacuations were continuing, the safety agency said.

Footage on Japanese TV showed the explosion had crumbled the building's walls, leaving only a skeletal metal frame standing. Its roof had also been blown off. Plumes of smoke spewed out of the plant, 20 miles from Iwaki.

The Japanese authorities have classified the event at Fukushima Daiichi Unit 1 as a level 4 "accident with local consequences" on the International Nuclear and Radiological Event Scale (INES). The scale is used to consistently communicate the safety significance of events associated with sources of radiation. The scale runs from 0 (deviation -- no safety significance) to 7 (major accident).

The 1979 Three Mile Island accident in Pennsylvania was a level 5 ("accident with wider consequences"). The 1986 Chernobyl disaster was a level 7 ("major accident).

Four workers were injured by the blast at Fukushima Unit 1, officials said.

Edano said the radiation around the plant had not risen after the blast, but had in fact decreased. He did not say why that was so.

Area residents were told to stay indoors, not to drink tap water and to cover their faces with masks or wet towels, according to Britain's Sky News.

Japanese authorities also told the U.N. atomic watchdog that they were making preparations to distribute iodine to people living near nuclear power plants affected by Friday's earthquake.

Officials were aware that the steam contained hydrogen, acknowledged Shinji Kinjo, spokesman for the government nuclear safety agency. More importantly, they also were aware they were risking an explosion by deciding to vent the steam.

The significance of the hydrogen began to come clear late Saturday:

— Officials decided to reduce rising pressure inside the reactor vessel, so they vented some of the steam buildup. They needed to do that to prevent the entire structure from exploding, and thus starting down the road to a meltdown.

— At the same time, in order to keep the reactor fuel cool, and also prevent a meltdown, operators needed to keep circulating more and more cool water on the fuel rods.

— Temperature in the reactor vessel apparently kept rising, heating the zirconium cladding that makes up the fuel rod casings. Once the zirconium reached 2,200 degrees Fahrenheit (1,200 Celsius), it reacted with the water, becoming zirconium oxide and hydrogen.

— When the hydrogen-filled steam was vented from the reactor vessel, the hydrogen reacted with oxygen, either in the air or water outside the vessel, and exploded.

A similar "hydrogen bubble" had concerned officials at the 1979 Three Mile Island nuclear disaster in Pennsylvania until it dissipated.

If the temperature inside the Fukushima reactor vessel continued to rise even more — to roughly 4,000 degrees Fahrenheit (2,200 Celsius) — then the uranium fuel pellets would start to melt.

According to experts interviewed by The Associated Press, any melted fuel would eat through the bottom of the reactor vessel. Next, it would eat through the floor of the already-damaged containment building. At that point, the uranium and dangerous byproducts would start escaping into the environment.

At some point in the process, the walls of the reactor vessel — 6 inches (15 centimeters) of stainless steel — would melt into a lava-like pile, slump into any remaining water on the floor, and potentially cause an explosion much bigger than the one caused by the hydrogen. Such an explosion would enhance the spread of radioactive contaminants.

If the reactor core became exposed to the external environment, officials would likely began pouring cement and sand over the entire facility, as was done at the 1986 Chernobyl nuclear accident in the Ukraine, Peter Bradford, a former commissioner of the U.S. Nuclear Regulatory Commission, said in a briefing for reporters.

Another expert, Ken Bergeron, a physicist and former Sandia scientist, added that as a result of such a meltdown the surrounding land would be off-limits for a considerable period of time, and "a lot of first responders would die."

'Not a fast reaction like at Chernobyl'TEPCO, which operates the six-reactor Daiichi site, said Friday that it had also lost cooling ability at a second reactor there and three units at its nearby Fukushima Daini site.

A "meltdown" is not a technical term. Rather, it is an informal way of referring to a very serious collapse of a power plant's systems and its ability to manage temperatures.

Yaroslov Shtrombakh, a Russian nuclear expert, said a Chernobyl-style meltdown was unlikely.

"It's not a fast reaction like at Chernobyl," he said. "I think that everything will be contained within the grounds, and there will be no big catastrophe."

In 1986, the Chernobyl nuclear reactor exploded and caught fire, sending a cloud of radiation over much of Europe. That reactor — unlike the Fukushima one — was not housed in a sealed container, so there was no way to contain the radiation once the reactor exploded.

Edwin Lyman, a senior scientist with the Union of Concerned Scientists, which opposes nuclear energy, told msnbc.com Friday that TEPCO was facing a potential catastrophe.

"What's critical is, are they able to restore cooling and prevent fuel damage? If the fuel starts to get damaged, eventually it will melt through the reactor vessel and drop to the floor of the containment building," raising the odds that highly radioactive materials could be released into the environment, he said.

msnbc.com

But Steve Kerekes, spokesman for the U.S.-based Nuclear Energy Institute, said that while the situation was serious, a meltdown remains unlikely and, even if it occurred would not necessarily pose a threat to public health and safety.

The Daiichi site is located in Onahama city. The 460-megawatt Unit 1 began operating in 1971 and is the oldest at the facility. It is a boiling water reactor that drives the turbine with radioactive water, unlike pressurized water reactors usually found in the United States. Japanese regulators decided in February to allow it to run another 10 years. The reactors were manufactured by GE. (Msnbc.com is a joint venture between NBC Universal and Microsoft. GE is a part owner of NBC Universal.)

The Fukushima Daiichi Nuclear Power Station has six reactors. Units 1, 2 and 3 were shut down due to the earthquake and units 4 to 6 were shutdown earlier for regular inspection, TEPCO said.

At Fukushima Daini Nuclear Power Station, the units 1 to 4 were shut down due to the earthquake, TEPCO said. A trapped crane operator there died Sunday, TEPCO said.

In a statement released Saturday, The Citizens' Nuclear Information Center said, "This could and should have been predicted ... We warned that Japan's nuclear plants could be subjected to much stronger earthquakes and much bigger tsunamis than they were designed to withstand."

Greenpeace called for the existing reactors to be phased out, saying in a statement, "How many more warnings before we finally grasp that nuclear reactors are inherently hazardous?"

Video: Meltdown fears at damaged nuclear reactor

Closed captioning of: Meltdown fears at damaged nuclear reactor

>>that. japanese officials say they have kal cau lighted that 160 people have been exposed to
radio activity
. 160,000 people have been evacuated from around two nuclear plants most from the one that suffered the explosion. more on this from nbc's
anne thompson
.

>> reporter: nothing frightened the world more than this. an explosion at the troubled fukushima one
power plant
. the
japanese government
declared emergency when the systems failed. now fear of a meltdown. throughout the day japanese tv reporters used diagrams and maps to help a near versus nation. the blast explosioned the ex exterior building and blamed the explosion on a buildup of hydrogen. this official said after the explosion the radiation leaking from the plant actually decreased as did the pressure.

>>i think they are being cautious and conservative and given the conditions there, i think that is wise.

>> reporter: when the earthquake struck, the reactor shut down and systems failed. without water to cool it, the core could over heat and melt down releasing
radio activity
into the environment. tonight, the japanese utility took the unusual step of pumping
sea water
into the reactor to prevent a melt down.

>>if you do so, you have decided that the consequences of not doing so are very serious. the evidence in this case is that they are worried about significant melting of the core.

>> reporter: more signs of concern. people living within the evacuation zone are scanned for radiation and the interational
atomic energy
agency said japan is preparing to give iodine to those living near the plant.

>> reporter: this was reported on a scale of 1 to 7. at pennsylvania's three mile island, there was a partial core melt down but not a significant release of radiation. japan now wait to see if it its fate is the same.
anne thompson
.

A radiation measuring instrument is seen next to some residents in Kawauchimura, a village within the 12- to 18-mile zone around the Fukushima Daiichi nuclear plant, on April 28. Most residents of Kawauchimura have evacuated in order to avoid the radiation, but some remain in the area of their own accord.
(Koichi Kamoshida / EPA)
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A brazier heats the house of Masahiro Kazami, located within a 12-mile radius of the Fukushima Daiichi nuclear plant, April 28.
(Koichi Kamoshida / EPA)
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Volunteers help clean a cemetery at Jionin temple in Ishinomaki, Miyagi prefecture, northeastern Japan, on April 29. Many volunteers poured into the disaster-hit region at the beginning of the annual Golden Week holiday.
(Hiro Komae / AP)
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Japanese government adviser Toshiso Kosako is overcome with emotion during a news conference on April 29 in Tokyo announcing his resignation. The expert on radiation exposure said he could not stay on the job and allow the government to set what he called improper radiation limits for elementary schools in areas near the tsunami-damaged Fukushima nuclear plant.
(AP)
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Farmer Tsugio Sato tends to his Japanese pear trees in Fukushima city, May 1. He said he expects to harvest the pears in October. Farmers and businesses face so-called "fuhyo higai," or damages stemming from the battered reputation of the Fukushima brand.
(Hiro Komae / AP)
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Members of Japan Ground Self-Defense Force in protective gear receive radiation screening in Minamisoma in Fukushima prefecture, after searching for bodies at an area devastated by the March 11 earthquake and tsunami.
(Reuters)
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Ruriko Sakuma, daughter of dairy farmer Shinji Sakuma, rubs a cow at their farm in the village of Katsurao in Fukushima prefecture on May 3. Thousands of farm animals died of hunger in the weeks following the quake.
(Yoshikazu Tsuno / AFP - Getty Images)
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A look at the worst earthquakes in recorded history, in loss of human life. (The March 11, 2011, earthquake and tsumani that affected eastern Japan is not included because the fatalities caused, about 15,000, are fewer than those resulting from the temblors listed below.) Sources: United States Geological Survey, Encyclopedia Britannica

1: Shensi, China, Jan. 23, 1556

Magnitude about 8, about 830,000 deaths.

This earthquake occurred in the Shaanxi province (formerly Shensi), China, about 50 miles east-northeast of Xi'an, the capital of Shaanxi. More than 830,000 people are estimated to have been killed. Damage extended as far away as about 270 miles northeast of the epicenter, with reports as far as Liuyang in Hunan, more than 500 miles away. Geological effects reported with this earthquake included ground fissures, uplift, subsidence, liquefaction and landslides. Most towns in the damage area reported city walls collapsed, most to all houses collapsed and many of the towns reported ground fissures with water gushing out.

1976: Workers start rebuilding work following earthquake damage in the Chinese city of Tangshan, 100 miles east of Pekin, with a wrecked train carriage behind them. (Photo by Keystone/Getty Images)

Magnitude 7.5. Official casualty figure is 255,000 deaths. Estimated death toll as high as 655,000.

Damage extended as far as Beijing. This is probably the greatest death toll from an earthquake in the last four centuries, and the second greatest in recorded history.

3: Aleppo, Syria, Aug. 9, 1138

Magnitude not known, about 230,000 deaths.

Contemporary accounts said the walls of Syria’s second-largest city crumbled and rocks cascaded into the streets. Aleppo’s citadel collapsed, killing hundreds of residents. Although Aleppo was the largest community affected by the earthquake, it likely did not suffer the worst of the damage. European Crusaders had constructed a citadel at nearby Harim, which was leveled by the quake. A Muslim fort at Al-Atarib was destroyed as well, and several smaller towns and manned forts were reduced to rubble. The quake was said to have been felt as far away as Damascus, about 220 miles to the south. The Aleppo earthquake was the first of several occurring between 1138 and 1139 that devastated areas in northern Syria and western Turkey.

4: Sumatra, Indonesia, Dec. 26, 2004

Getty Images
/
Getty Images

MEULABOH, INDONESIA - DECEMBER 29: In this handout photo taken from a print via the Indonesian Air Force, the scene of devastation in Meulaboh, the town closest to the Sunday's earthquake epicentre, is pictured from the air on December 29, 2004, Meulaboh, Aceh Province, Sumatra, Indonesia. The western coastal town in Aceh Province, only 60 kilometres north-east of the epicentre, has been the hardest hit by sunday's underwater earthquake in the Indian Ocean. Officials expected to find at least 10,000 killed which would amount to a quarter of Meulaboh's population. Three-quarters of Sumatra's western coast was destroyed and some towns were totally wiped out after the tsunamis that followed the earthquake. (Photo by Indonesian Air Force via Getty Images)

Magnitude 9.1, 227,898 deaths.

This was the third largest earthquake in the world since 1900 and the largest since the 1964 Prince William Sound, Alaska temblor. In total, 227,898 people were killed or were missing and presumed dead and about 1.7 million people were displaced by the earthquake and subsequent tsunami in 14 countries in South Asia and East Africa. (In January 2005, the death toll was 286,000. In April 2005, Indonesia reduced its estimate for the number missing by over 50,000.)

Magnitude 7.0. According to official estimates, 222,570 people killed.

According to official estimates, 300,000 were also injured, 1.3 million displaced, 97,294 houses destroyed and 188,383 damaged in the Port-au-Prince area and in much of southern Haiti. This includes at least 4 people killed by a local tsunami in the Petit Paradis area near Leogane. Tsunami waves were also reported at Jacmel, Les Cayes, Petit Goave, Leogane, Luly and Anse a Galets.

6: Damghan, Iran, Dec. 22, 856

Magnitude not known, about 200,000 deaths.

This earthquake struck a 200-mile stretch of northeast Iran, with the epicenter directly below the city of Demghan, which was at that point the capital city. Most of the city was destroyed as well as the neighboring areas. Approximately 200,000 people were killed.

7: Haiyuan, Ningxia , China, Dec. 16, 1920

7.8 magnitude, about 200,000 deaths.

This earthquake brought total destruction to the Lijunbu-Haiyuan-Ganyanchi area. Over 73,000 people were killed in Haiyuan County. A landslide buried the village of Sujiahe in Xiji County. More than 30,000 people were killed in Guyuan County. Nearly all the houses collapsed in the cities of Longde and Huining. About 125 miles of surface faulting was seen from Lijunbu through Ganyanchi to Jingtai. There were large numbers of landslides and ground cracks throughout the epicentral area. Some rivers were dammed, others changed course.

The memories of the massive Damghan earthquake (see above) had barely faded when only 37 years later, Iran was again hit by a huge earthquake. This time it cost 150,000 lives and destroyed the largest city in the northwestern section of the country. The area was again hit by a fatal earthquake in 1997.

9: Kanto, Japan, Sept. 1, 1923

Hulton Archive
/
Getty Images

1923: High-angle view of earthquake and fire damage on Hongokucho Street and the Kanda District, taken from the Yamaguchi Bank building after the Kanto earthquake, Tokyo, Japan. (Photo by Hulton Archive/Getty Images)

7.9 magnitude, 142,800 deaths.

This earthquake brought extreme destruction in the Tokyo-Yokohama area, both from the temblor and subsequent firestorms, which burned about 381,000 of the more than 694,000 houses that were partially or completely destroyed. Although often known as the Great Tokyo Earthquake (or the Great Tokyo Fire), the damage was most severe in Yokohama. Nearly 6 feet of permanent uplift was observed on the north shore of Sagami Bay and horizontal displacements of as much as 15 feet were measured on the Boso Peninsula.

10: Ashgabat, Turkmenistan, Oct. 5, 1948

7.3 magnitude, 110,000 deaths.

This quake brought extreme damage in Ashgabat (Ashkhabad) and nearby villages, where almost all the brick buildings collapsed, concrete structures were heavily damaged and freight trains were derailed. Damage and casualties also occurred in the Darreh Gaz area in neighboring Iran. Surface rupture was observed both northwest and southeast of Ashgabat. Many sources list the casualty total at 10,000, but a news release from the newly independent government on Dec. 9, 1988, advised that the correct death toll was 110,000. (Turkmenistan had been part of the Soviet Union, which tended to downplay the death tolls from man-made and natural disasters.)